This invention relates to microwave travelling wave tube amplifiers (TWTAs) and more particularly, it relates to a linearizer for super high frequency (SHF) travelling wave tube amplifiers for use in the uplink of a ground terminal.
Distortion compensation techniques are necessary to reduce amplifier-produced distortion, such as amplitude modulation to phase modulation conversion (AM/PM) and amplitude modulation to amplitude modulation conversion (AM/AM), in the transfer characteristics of amplifiers, and in particular wideband amplifiers operating at frequencies where band-limiting feedback techniques are not suitable. Accordingly, it is desirable to provide a predistortion network, or linearizer, which does not substantially reduce the available bandwidth of a microwave amplifier such as a travelling wave tube amplifier.
Various predistortion compensation techniques have been employed in order to improve operation of broadband amplifiers. In the past, feedforward and feedback arrangements have been suggested in order to duplicate and compensate generally for nonlinear transfer characteristics of travelling wave tube amplifiers. It has been found, however, that such generalized techniques are not suited under conditions requiring use of the broadest possible bandwidth. Moreover, each travelling wave tube amplifier (TWTA) has a frequency dependent characteristic which cannot be generalized. Therefore, what is needed is a linearizer circuit which is readily adjustable, that is trimmed, to the precise characteristics of the TWTA with which it is intended to be used. Moreover, it is desirable, and by this invention as hereafter explained, possible to compensate for the TWTA saturation characteristics at the desired band center without compromising performance of the TWTA at the band edges.
The following patents were uncovered in a search of the prior art in respect to the subject invention:
Travis, U.S. Pat. No. 4,453,133 issued June 5, 1984, for "Active Predistorter for Linearity Compensation". This invention employs predistortion components in connection with a wideband amplifier. In this particular implementation, it is believed that the circuit would be impractical to develop an active microwave device which matches the characteristics of the device to be compensated and further, no provision is made for AM/PM compensation.
Whartenby and Kumar, U.S. Pat. No. 4,553,514, describes an active predistortion circuit which makes it feedback schemes at microwave frequencies, which are sometimes desirable, virtually impossible to implement. Moreover, no provision is made for independent AM/AM and AM/PM compensation, and no provision is made to address preexisting amplitude and phase non-linearities.
Kumar and Whartenby, U.S. Pat. No. 4,564,816, describes a further active predistortion circuit. As with other active device-based realization, it is less likely to be reliable and accurate under all operating conditions and temperatures. Moreover, no provision is made for independent AM/AM and AM/PM compensation, and no provision is made to address preexisting amplitude and phase non-linearities.
Green, Jr., et al., U.S. Pat. No. 4,532,477, describes a still further active predistortion amplifier which likewise teaches away from the use of passive predistortion and lacks independent AM/AM and AM/PM compensation.
Heynisch, U.S. Pat. No. 4,529,945, describes a method for removing distortion which also involves active elements. Again, no provision is made for independent AM/AM and AM/PM compensation. The circuit calls for use of matching predistorter diodes, a difficult and typically expensive requirement.
Huang et al., U.S. Pat. No. 4,465,980, is a mixed passive and active predistortion network. However, operation is geared to relatively low frequency operation. Again, no provision is made for independent AM/AM and AM/PM compensation.
Nojima et al., U.S. Pat. No. 4,329,655, describes an adaptive equalization linearizer. The circuit is of limited usefulness in that it is designed to reduce noise caused by non-linearities in connection with a noise detection scheme.
Davis et al., U.S. Pat. No. 4,291,277, describes a complex digitally-controlled predistortion circuit which adapts to time-varying characteristics of a high power microwave amplifier. Such a system is expensive to implement and so complex that it is difficult to analyze circuit operation and predict performance.
Satoh, U.S. Pat. No. 4,283,684, describes a compensating circuit for high frequency amplifiers. The invention described is an active circuit which must be matched to the device to be linearized, it is difficult to analyze and build because it contains an asymmetrically and reactively loaded hybrid. No known analytical technique can be used to design such a circuit, so expensive cut and try techniques are required to realize such a circuit. The invention has limited use and mainly is an application of a microwave integrated circuit for use in a satellite.
Heiter et al., U.S. Pat. No. 4,122,399, describes a distortion generator. The predistortion elements are reactive, so large signal effects may cause significant bias offsets and instabilities. Coupled mismatches may also cause parametric oscillations and generate excessive harmonic content, and thus may be self-defeating as an addition to an amplifier circuit.
Sato et al., U.S. Pat. No. 4,068,186, describes a passive predistortion network. The circuit has an asymmetrically and reactively loaded hybrid. Implementation based on analytic models is difficult and may require costly cut-and-try methods of realization. The adjustment capabilities are inadequate, since there is no independent control of AM/AM and AM/PM characteristics. The circuit is better suited for a relatively narrower bandwidth is contemplated for the present invention.